Vapor grown carbon fiber and carbon nanotubes (unless otherwise specified, both vapor grown carbon fiber and carbon nanotubes are hereinafter collectively referred to as “carbon nanotubes”) have been extensively studied in terms of their applications in, for example, field emitters, transistors, sensors, hydrogen occlusion, conductive plastics, fuel cells and solar cells.
Currently, carbon nanotubes are generally produced through the CVD method employing a raw material gas such as methane; the arc discharge method employing a solid raw material such as graphite (Japanese Patent Application Laid-Open (kokai) No. 2004-256373); or the laser ablation method. Carbon nanotubes produced through any of these methods are expensive, because of high costs for raw materials and apparatuses.
In the future, when carbon nanotubes come to be used in large amounts, a low-cost method for the synthesis therefor will definitely be needed. In addition, natural resources present in the natural world and recycled raw materials will come to be employed from environmental considerations. Furthermore, oils that are consumed via combustion are required to be fixed as carbon nanotubes, from the viewpoint of reduction in amount of carbon dioxide emitted to the atmosphere.
Among various production methods for carbon nanotubes, the CVD method—a type of vapor growth technique—is the lowest-cost method for producing carbon nanotubes. However, since the production cost is still unsatisfactory and remains to be further reduced, there is a demand for a carbon nanotube production method which allows mass production of carbon nanotubes at lower cost.
FIG. 1 is a schematic representation of an example reactor for continuously producing carbon nanotubes through vapor growth. In one typical procedure for producing carbon nanotubes, a hydrocarbon such as CO, methane, acetylene, ethylene, benzene or toluene is employed as a feedstock. In the case where the feedstock hydrocarbon assumes the gaseous state at room temperature, the hydrocarbon is mixed with a carrier gas, thereby serving as a feedstock. In the case where the hydrocarbon assumes the liquid state, the liquid is vaporized, and then mixed with a carrier gas, thereby serving as a feedstock. Alternatively, liquid hydrocarbon may be sprayed into a heating zone. As a catalyst, a supported catalyst where a metal is supported on a support such as alumina or an organometallic compound such as ferrocene is used. When a supported catalyst is employed, the catalyst is placed in a reaction zone in advance and heated, and is subjected to essential preliminary treatment. Subsequently, a feedstock hydrocarbon is supplied to the catalyst for reaction (as illustrated in FIG. 1). Alternatively, reaction may also be carried out by feeding in a continuous or pulse-like manner from the outside to the reactor, a supported catalyst that has been preliminarily treated. In a still alternative procedure, a feedstock hydrocarbon and an organometallic compound such as ferrocene, which is a homogeneous catalyst precursor compound, are fed to the heating zone in a continuous or pulse-like manner, and the catalyst precursor compound is thermally decomposed to form metallic particles serving as a catalyst, whereby carbon nanotubes can be formed in the presence of the catalyst. The thus-formed product is collected by a collector disposed at the outlet of the heating zone or inside the heating zone. After the collected product has been subjected to a reaction for a predetermined period of time, the product is recovered.
Carbon nanotube production methods employing vapor phase growth are generally classified into the following two types:
(a) a method in which a substrate or a boat formed of alumina or graphite which supports a catalyst or a precursor compound thereof is placed in a heating zone, and the catalyst or the precursor is brought into contact with a hydrocarbon gas fed from a vapor phase (Chemical Physics Letters, 384 (2004), 98-102); and
(b) a method in which a metallocene or a carbonyl compound is dissolved in a liquid hydrocarbon to serve as a catalyst precursor, and the hydrocarbon solution containing the catalyst precursor compound is fed to a heating zone, whereby the catalyst is brought into contact with the hydrocarbon at high temperature (Japanese Patent Application Laid-Open (kokai) No. 2004-176244).
According to the above method (a), carbon nanotubes can be produced at a relatively low temperature of 1,000° C. or lower. However, percent conversion of hydrocarbon gas to carbon nanotubes is low, resulting in an increase in raw material cost, which is problematic.
Hereinafter, the term “percent conversion” refers to a value obtained by dividing the amount of collected solid by the amount of raw material used.
In the case of the above method (b), a hydrocarbon such as benzene or toluene is generally employed. When such a hydrocarbon is used, percent conversion reaches 50% or higher, which is a comparatively high value. However, reaction must be carried out at a temperature as high as 1,000° C. or higher, leading to an increase in fuel and facility costs.
Since these two methods employ flammable gases such as hydrogen serving as a carrier gas and hydrocarbon gas, limitations are imposed on the material and structure of production apparatuses, which elevates costs.
The hydrocarbons employed as carbon sources are all produced from fossil fuels, which is not preferred from the viewpoint of environmental issues. If natural carbon sources present in the natural world and recycled raw materials can be employed, environmental load can be reduced.